Patent Document 1 discloses a thermochromic writing instrument storing a writing body longitudinally movably in a barrel, provided with an operation unit on an outer surface of the barrel, configuring a nib of the writing body capable of appearing from a front end hole of the barrel by operating the operation unit, storing a thermochromic ink inside the writing body, provided with a nib capable of ejecting the thermochromic ink at a front end of the writing body, and provided with a friction unit, on an outer surface of the barrel, rubbing handwriting in the thermochromic ink and capable of thermally changing the color of the handwriting by frictional heat generated at that time.
In Patent Document 1 above, in a case of providing a friction unit in an operation unit at a rear end of the barrel in a type to push the operation unit forward, when friction operating using the friction unit, there is a risk of moving the operation unit forward by a written surface and thus not allowing a stable friction operation. In particular, when adopting an appearance mechanism of a type of push operating the operation unit forward (so-called double knock type) for both a nib projection operation and a nib retraction operation and also providing the friction unit in the operation unit, there is a risk of longitudinally shaking an operation body in a nib projection condition and thus not allowing a stable friction operation.
FIG. 12 in Patent Document 1 discloses a configuration provided with a tubular operation unit longitudinally movably, provided with a projected clip, capable of sandwiching a pocket or the like, on an outer surface of the tubular operation unit, and provided with a friction unit on an outer surface at the rear end of the tubular operation unit. However, since a thermochromic writing instrument of this configuration pushes the friction unit at the time of a nib appearance operation, there is a risk of contaminating the friction unit with hand grime and the like. Then, in a case of rubbing handwriting in a thermochromic ink with the friction unit in a contaminated condition, there is a risk of contaminating the written surface (for example, paper surface) having the handwriting.
FIG. 15 in Patent Document 1 discloses a configuration provided with an appearance mechanism of a configuration to put from a nib retraction condition into a nib projection condition (so-called side sliding multi-cartridge type) by having an operation unit projected radially outward from a side wall of a barrel and by push operating the operation unit forward against a backward bias, and provided with a friction unit on an outer surface at a rear end of the barrel. However, in a thermochromic writing instrument of this configuration, when putting from a nib projection condition into a nib retraction condition (when releasing the nib projection condition), an operation body attached to another writing body has to be operated. Therefore, a plurality of operation bodies (that is, a plurality of projections) is required to be provided, and the degree of freedom for appearance design is reduced. Further, when the barrel is provided with a clip capable of sandwiching a pocket or the like, projections increase even more and the degree of freedom for appearance design is reduced.    Patent Document 1: WO 2008/105227 publication